feat(dev-infra): add logic for determining active LTS branches (angular#38656)

Adds logic for determining active LTS branches for a given
release configuration. The active LTS branches can be determined
by querying NPM and matching dist tags against a specific
pattern. i.e. `v{major}-lts`.

This logic will be useful for the release tool that supports
publishing of active LTS version branches.

PR Close angular#38656

Author: devversion

dev-infra/release/versioning/long-term-support.ts

@@ -6,6 +6,30 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import * as semver from 'semver';
+
+import {ReleaseConfig} from '../config/index';
+
+import {fetchProjectNpmPackageInfo} from './npm-registry';
+
+/** Interface describing determined LTS branches. */
+export interface LtsBranches {
+  /** List of active LTS version branches. */
+  active: LtsBranch[];
+  /** List of inactive LTS version branches. */
+  inactive: LtsBranch[];
+}
+
+/** Interface describing an LTS version branch. */
+export interface LtsBranch {
+  /** Name of the branch. */
+  name: string;
+  /** Most recent version for the given LTS branch. */
+  version: semver.SemVer;
+  /** NPM dist tag for the LTS version. */
+  npmDistTag: string;
+}
+
 /**
  * Number of months a major version in Angular is actively supported. See:
  * https://angular.io/guide/releases#support-policy-and-schedule.
@@ -18,6 +42,44 @@ export const majorActiveSupportDuration = 6;
  */
 export const majorActiveTermSupportDuration = 12;
 
+/** Regular expression that matches LTS NPM dist tags. */
+export const ltsNpmDistTagRegex = /^v(\d+)-lts$/;
+
+/** Finds all long-term support release trains from the specified NPM package. */
+export async function fetchLongTermSupportBranchesFromNpm(config: ReleaseConfig):
+    Promise<LtsBranches> {
+  const {'dist-tags': distTags, time} = await fetchProjectNpmPackageInfo(config);
+  const today = new Date();
+  const active: LtsBranch[] = [];
+  const inactive: LtsBranch[] = [];
+
+  // Iterate through the NPM package information and determine active/inactive LTS versions with
+  // their corresponding branches. We assume that a LTS tagged version in NPM belongs to the
+  // last-minor branch of a given major (i.e. we assume there are no outdated LTS NPM dist tags).
+  for (const npmDistTag in distTags) {
+    if (ltsNpmDistTagRegex.test(npmDistTag)) {
+      const version = semver.parse(distTags[npmDistTag])!;
+      const branchName = `${version.major}.${version.minor}.x`;
+      const majorReleaseDate = new Date(time[`${version.major}.0.0`]);
+      const ltsEndDate = computeLtsEndDateOfMajor(majorReleaseDate);
+      const ltsBranch: LtsBranch = {name: branchName, version, npmDistTag};
+      // Depending on whether the LTS phase is still active, add the branch
+      // the list of active or inactive LTS branches.
+      if (today <= ltsEndDate) {
+        active.push(ltsBranch);
+      } else {
+        inactive.push(ltsBranch);
+      }
+    }
+  }
+
+  // Sort LTS branches in descending order. i.e. most recent ones first.
+  active.sort((a, b) => semver.rcompare(a.version, b.version));
+  inactive.sort((a, b) => semver.rcompare(a.version, b.version));
+
+  return {active, inactive};
+}
+
 /**
  * Computes the date when long-term support ends for a major released at the
  * specified date.